It is unknown whether beta cell dysfunction and insulin resistance in Gestational Diabetes Mellitus (GDM) is representative of a chronic maternal defect, unmasked by pregnancy, or whether it is the result of an imbalance of a placental hormones. Undiscovered placental factors which vary between pregnancies likely contribute to the pathogenesis of GDM. To elucidate the pathophysiology underlying GDM, the investigators will attempt to discover these factors and characterize pregnancy-associated changes in insulin secretion and sensitivity in women with and without GDM.